Reading problems have negative consequences such as school failure and are associated with behavior problems that can adversely impact children's health and wellbeing. In Project IV, we will use an ethnically/racially/economically diverse sample of 866 identical and 1,725 fraternal twin pairs in grades K-6 to accomplish three specific aims that will significantly advance our understanding of reading problem phenotypes, the environmental context in which genetic risk for reading problems may be expressed, and genetic and environmental factors underlying status and stability of comorbid reading and behavior problems. For aim 1, reading measures from Florida's Progress Monitoring and Reporting Network (PMRN) will be used to create classifications of reading problems in order to identify those that have substantial genetic influence. Behavioral, cognitive, and school/home environment data collected in year 1 ofthe project on a subsample of 350 identical and 350 same-sex fraternal twins will be used to created composites of salient environmental factors (e.g., home literacy, school resource level, classroom effectiveness) and behavior problems related to executive cognitive dysfunction (e.g., attention, impulsivity, etc.). For aim 2, gene-environment interplay processes underlying reading problems will be identified by testing for moderating effects of salient environmental factors on genetic and environmental variance associated with reading measures from the PMRN (e.g., word reading fluency, comprehension) using biometrical models. For aim 3, the subsample of 700 twin pairs will be re-assessed on behavior and environment measures in years 3 and 5 ofthe project and those data will be combined with PMRN reading data on the twins to estimate genetic and environmental contributions to status and stability ofthe comorbidity between reading problems and behavior problems using bivariate biometrical models.